Default Static Route

A default static route is a manually configured route that forwards traffic destined for unknown networks to a specific next-hop address.

Think of the default static route as a route used when no other routes in your router can be used to send the packet to the destination. (If you’re not yet familiar with regular static routes, see our Static Route lesson first.)

Let’s look at this example together: we have a packet with destination IP 8.8.8.8, and our router needs to find a route to send this packet to its destination.

Figure 1 – Default Static Route Lookup

There are three routes in this routing table. The first route is the default route with subnet 0.0.0.0 and mask /0, which means: "you can use this route when no other route matches the destination IP," because 0.0.0.0/0 includes all IP addresses.

The second route is 192.168.1.0/24, which does not match the destination IP.
The third route is 192.168.2.0/24, which also does not match.

Longest Prefix Match (LPM)

The router always prefers the most specific route (longest prefix match).
0.0.0.0/0 is used only if no other route matches the destination.

Since no other route matches, the router will use the default static route and forward the packet to the next hop 203.0.115.2 via G0/0.

🔻 What Happens Without a Default Route

Let’s take a realistic example where no default route is configured, PC1 sends a packet to destination IP 8.8.8.8 again and you can see below our router R1 dropped the packet because no entry in its routing table was able to match the destination.

Figure 2 – Packet dropped due to missing default route

In Cisco IOS routers, configuring a default static route is pretty easy, the command is ip route 0.0.0.0 0.0.0.0 .

In the example below, R1 is connected to the Internet Service Provider and a default static route can be used to send the unknown destination to the ISP. To do this, we specify the next hop as the IP interface of the internet service provider.